| Code | Name | Region |
|---|---|---|
| USOFU | Olsburg | KS |
| USOFO | Old Fort | OH |
| USOFJ | Old Fort | TN |
| USO7G | Old Glory | TX |
| USOGW | Old Greenwich | CT |
| USOLH | Old Harbor | AK |
| USOHI | Old Hickory | AR |
| USXKA | Old Hickory | TN |
| USOLL | Old Lyme | CT |
| USOZI | Old Monroe | MO |
| USLDC | Old Ocean | TX |
| USOO2 | Old Orchard Beach | ME |
| USDKH | Old Rock Hill | GA |
| USOSY | Old Saybrook | CT |
| USOPN | Old Tappan | NJ |
| USOLD | Old Town | ME |
| USOW9 | Old Westbury | NY |
| USOH2 | Oldham | SD |
| USABA | Oldsmar | FL |
| USOXE | Oldtown | MD |
| USOLE | Olean | NY |
| USOKO | Oklahoma | TX |
| USOKC | Oklahoma City | OK |
| USOKJ | Oklawaha | FL |
| USOKM | Okmulgee | OK |
| USO2K | Okoboji | IA |
| USOKA | Okolona | MS |
| USOKZ | Okolona | OH |
| USO3L | Ola | AR |
| USZE3 | Olancha | CA |
| USOXT | Olanta | PA |
| USFIZ | Olar | SC |
| USOZT | Olathe | CO |
| USOLB | Olathe | KS |
| USOHA | Olathia | MO |
| USOPG | Old Bethpage | NY |
| USODR | Old Bridge | NJ |
| USDXX | Old Brookville | NY |
| USOC6 | Old Chatham | NY |
| USXJZ | Old Forge | PA |
| USOFT | Old Fort | NC |
| USOHY | Ohiopyle | PA |
| USOHV | Ohioview | PA |
| USOYX | Ohkay Owingeh | NM |
| USOTY | Oil City | LA |
| USOIL | Oil City | PA |
| USODC | Oildale | CA |
| USIUN | Oilton | OK |
| USITB | Oilton | TX |
| USOVB | Oilville | VA |